Advanced composites are high strength, high modulus materials which are finding increasing use as structural components in aircraft, automotive, and sporting goods applications. Typically they comprise structural fibers such as carbon fibers in the form of woven cloth or continuous filaments embedded in a thermosetting resin matrix.
Most advanced composites are fabricated from prepreg, a ready-to-mold sheet of reinforcement impregnated with uncured or partially cured resin. Resin systems containing an epoxide resin and aromatic amine hardener are often used in prepreg since they possess the balance of properties required for this composite fabrication process. State-of-the-art epoxy/carbon fiber composites have high compressive strengths, good fatigue characteristics, and low shrinkage during cure. However, most epoxy formulations absorb moisture which reduces their high temperature properties. As a result they are not suitable for use at 350.degree. F. or greater in a moisture saturated condition. There is therefore a need for resin systems which afford composites which can retain a high level of properties at 350.degree. F. under such moisture saturated conditions.
Most prepreg resins designed for use at 350.degree. F. are made by combining bismaleimides of Formula I with liquid coreactants containing other reactive groups such as amines, epoxides, cyanates or comonomers containing --CH.dbd.CH.sub.2, ##STR1## or --CH.dbd.CH-- groups which can react or polymerize with the carbon-carbon double bonds of the maleimide groups. ##STR2##
In common bismaleimides, R is the residue of an aromatic diamine such as methylene dianiline or m-phenylene diamine.
However prior to the present invention the available aromatic bismaleimides usually contained one, two or four aromatic rings in the structure. Few, if any, bismaleimides have been reported containing three aromatic rings in the molecule, primarily due to a lack of available diamine precursors containing three aromatic rings. Accordingly, it was found that novel bismaleimides could be prepared which contained three aromatic rings in the structure and that these novel bismaleimides, in some respects, optimized the desirable characteristics of both the one or two and four aromatic ring-containing bismaleimides.
For example, bismaleimides containing one or two aromatic rings tend to give formulations which are characterized by high water absorption, high Tg and low toughness. Equivalent formulations in which the aromatic nucleus has four, or more aromatic rings, tend to be characterized by lower water absorption, low Tg, and higher toughness. It is desirable to provide resins characterized by high Tg, low water absorption, and high toughness.